1. Field of the Invention
The present invention relates to a power storage device including a nonaqueous electrolyte secondary battery, particularly a lithium ion secondary battery (hereinafter simply referred to as a lithium ion battery).
2. Description of the Related Art
As a secondary battery with large storage capacity, a lithium ion battery is mounted not only on a small portable electric appliance but also on an electric vehicle or the like these days. For a positive electrode of a lithium ion battery, lithium cobaltate (LiCoO2) has been conventionally used.
However, since cobalt is a scarce resource, a secondary battery using LiCoO2 or the like as a positive electrode active material is difficult to mass-produce and increase its size for an electric vehicle battery and the battery is inevitably highly expensive. For this reason, as a positive electrode active material, instead of cobalt, attempts have been made to use a lithium-iron composite oxide having an olivine structure and using iron that is an inexpensive abundant resource as its main constituent element (e.g., lithium iron phosphate (LiFePO4)) or a lithium-manganese composite oxide (e.g., lithium manganese phosphate (LiMnPO4)) (see Patent Document 1).
In addition, research on the physical properties of lithium iron phosphate has been carried out and the magnetic properties of a single crystal of lithium iron phosphate have been studied (see Non-Patent Document 1).